Fish and seafood comprise a significant portion of the diet of nearly every culture throughout the world. In the 1990's annual global per-capita consumption of seafood exceeded 20 lbs (15 lbs US). This level of consumption corresponds to more than five hundred million tons of seafood being utilized on an annual basis. These levels have continued to increase in the early 2000's. Convenience and the availability of seafood products in inland areas necessitate proper storage and transport mechanisms. While rapid freezing, refrigeration, and advanced handling and processing techniques have greatly improved capacity for delivering high quality products to the consumer, there is presently no available mechanism by which distributors, wholesalers, retailers, and consumers can be assured of product freshness at the point of purchase. Deterioration of seafood during storage not only results in a reduction of quality in the food product but has significant economic and health issues. In the area of health concerns, the proliferation of bacteria in the flesh of fish and shellfish during storage lead to many forms of food-borne illness. One of the best examples is scombroid poisoning. Scombroid poisoning results from the proliferation of bacteria in the flesh of many types of fish including, abalone, amberjack, bluefish, mackerel, mahi mahi, sardines and tuna. Excreted bacterial decarboxylase enzymes act on histidine and other amino acids in fish flesh producing large quantities of histamine and other toxic byproducts. No form of end-stage processing or high temperature cooking can counteract the hazardous nature of the toxins. Scombroid poisoning remains one of the most common forms of fish poisoning, even while most incidents go unreported due to confusion of symptoms with those of other illnesses. Even with this low level of recognition, more than 5,000 cases were recorded in US and Japan over a twelve year period. Scombroid poisoning is but one of dozens of food borne illnesses that can be directly attributed to improper or extended periods of seafood storage.
As consumers are faced with media reports of illnesses resulting from seafood consumption, the seafood industry must make strides to address consumer concerns. One such mechanism is the use of expiration dating on packaging materials. These dates are based upon research data for different products and are directly related the Manufacturer's Date Packaged (DP) code. In the US, the FDA requires expiration dating on all seafood products that reflect recommendations for maximum freshness and nutrient value. These dating guidelines are based upon proper storage conditions and handling techniques. While expiration dating has merit and has undoubtedly reduced consumption of spoiled food products, dating can not take into account improper storage and handling nor can it be expected to be 100% accurate on a batch to batch basis. Expiration dating has an economic impact as well. In 1998 fish and seafood sales in the US supermarkets reached more than $3.8 billion. The value of those seafood products that were disposed of due to expiration dating was more than $350 million or nearly 10% of total seafood sales. No data is readily available to determine the percentage of disposed seafood that was still viable at the time of disposal.
Clearly a simple, low-cost technique to indicate the freshness level of seafood products would have great value. It could further reduce incidences of food-borne illness, rest consumer confidence in seafood quality and may improve the economics of seafood sales by reducing losses due to expiration to a bare minimum. The industry has begun to take steps to develop such a product. Several supermarket chains have begun using “freshness tags” within seafood packaging. Freshness tags are color-producing materials that undergo a color change when seafood packages are held or transported outside a fixed temperature range for an extended period of time. Freshness tags are a first step to ensuring that expiration dating has validity and provide positive feedback to the consumer. Unfortunately, tags are of limited practical utility as they have not been present with the seafood since the date of catch or harvest. They also can not provide any useful information on a sample to sample basis.
There are a number of techniques available to assess fish quality. The most common approach involves sensory methods to evaluate food characteristics by sight, smell, and touch. Trained individuals can be quite adept at evaluating seafood quality. While it is clear that sensory assessment of fish has utility, proper analysis can only be done by a well trained group of three to six assessors. While the basics of assessment can be learned during a 2-day training course, efficiency can only be gained through years of experience. The average consumer obviously does not have the aptitude to properly and critically evaluate seafood in the same manner as a trained professional.
Much research has gone into correlating sensory assessments to chemical and bacteriological laboratory data. A viable cell count after incubation of fish flesh or direct microscopic analyses of food products are common approaches to assessing the degree of bacterial activity on a seafood product. Drawing a correlation between bacterial activity and spoilage is non-trivial, as much of the bacterial flora present on fish has no impact on spoilage. An accurate assessment of “spoilage potential” must be conducted to give viable cell counts definitive meaning. Unfortunately laboratory assessments are expensive, time consuming, and usually destroy the food sample. Typical chemical analysis involves either extraction of seafood with organic solvents and subsequent GC/HPLC analysis and identification of off-gassing volatile chemicals. Simultaneous chemical analyses during microbial cell counts have shown that many chemical markers are indicative of microbial contamination and can potentially be used to track spoilage. The literature is full of reports describing correlations between different chemical markers and seafood quality. These markers include a variety of amines, hypoxanthine, trimethylamine, ammonia, total volatile bases, ethanol, histamine, and hydrogen sulfide. Unfortunately, there are no clear trends in the literature and one study often contradicts others with respect to which chemical indicators provide correlations to quality in multiple species.